Conventional techniques for characterizing the geometry of bed boundaries rely on dipmeter logging tools such as the Stratigraphic High Resolution Dipmeter Tool(SHDT) of Schlumberger, disclosed in U.S. Pat. No. 4,251,773 of Michel Cailliau Asnieres and Philippe Vincent, issued Feb. 17, 1981. This dipmeter tool has four pads equiangularly distributed about the axis of the tool and adapted to engage the borehole wall at 90.degree. intervals. Each pad carries electrical sensors which yield resistivity measurements pertaining to the respective sectors of the borehole wall engaged by the pads. As the tool is moved along the borehole wall, the sensors continuously measure the resistivity of the adjacent formation, and logs are produced which show the variations in resistivity along the longitudinal direction of the borehole. Sharp variations in resistivity indicate boundaries between different beds in the earth formation. The logging data produced by the pads at the different angular positions of the pads are processed to provide information as to the dip of bed boundaries, i.e., the orientations of the bed boundaries with respect to a terrestrial reference. Known techniques include the correlation of the resistivity data from the several pads, as described in the above-identified patent.
A major advance in borehole logging technique has been the development by Schlumberger of the Formation Microscanner ("FMS"), a borehole imaging system. This system uses a tool having wall-engaging pads each carrying a dense array of electrical sensors distributed in the circumferential direction with respect to the borehole. The voltages at these sensors are sampled electrically as the tool moves along the borehole, to produce image data in electrical form which can be processed and rendered visible, as by photographic print-out or cathode-ray tube display for example, thereby forming a two-dimensional visible image of the resistivity over the logged segments of the borehole walls. Because of the high density of sensors in the circumferential direction, and the high rate of sampling in the longitudinal direction, the images thus obtained exhibit a resolution on the order of 0.5 cm, allowing very fine details of the formation to be distinguished. The FMS and associated processing techniques are described in U.S. Pat. Nos. 4,468,623 of Stanley C. Gianzero, David E. Palaith and David S.K. Chan, issued Aug. 28, 1984, and U.S. Pat. No. 4,567,759 of Michael P. Ekstrom and David S.K. Chan, issued Feb. 5, 1986. In such images, bed boundaries can be visually identified, since the sharp changes in resistivity at boundaries of the beds translate into sharp visible contrasts in the images.
An object of the present invention is to provide new and useful method and apparatus for processing borehole image data to produce data delineating bed boundaries.
Another object is to derive, from borehole image data, dip information with respect to the planes of the beds.